Electron beam converging device



MlcHlAKl lTo 3,013,172

ELECTRON BEAM CONVERGING DEVICE 2 Sheets-Sheet l Dec. 12, 1961 Filed Jan. 29. 1959 F/G. XO. Prior A/4/1!) Inventor ttorney Dec. 12, 1961 MlcHlAKi lTo 3,013,172

ELEcTRoN BEAM coNvERGING DEVICE Filed Jan. 29. 1959 2 Sheets-Sheet 2 MAGNETIC MA-GNETK;

5 MATERI F/G. 30. MATERIAL (l ,2/ 9 ,2 f"

/ fw V w MM I 2 .4 -4 L:

/ w w W Y 5% M7A s/v/s/v/v}s(/v ss/vs/v/v /vs 7' NO -MAGNETIC MAGNETIC MATERIAL MATERIAL F/G.3b. 52 lo i I WM /V Inventor M. ITO

By L? ttorney United States Patent ELECTRON BEAM CONVERGING DEVICE Michiaki Ito, Tokyo, Japan, assigner to Nippon Electric fompany, Limited, Tokyo, Japan, a corporation of apan Filed `Ian. 29, 1959, Ser. No. '789,846 Claims priority, application Japan Feb. 25, 1958 3 Claims. (Cl. 313-84) This invention relates to a beam focussing device which makes use of periodically varying magnetic iiux as the converging means for the electron beam. For example, it relates to an electron beam converging device used in microwave beam tubes, particularly traveling wave tubes.

Electron beam converging devices previously proposed attempt to provide a magnetic flux along the axis of the beam which varies in strength substantially as a sine wave. This is done by arranging ring shaped magnets in opposition, that is with the like poles adjacent one another. Generally, also, there are provided magnetic pole pieces between the adjacent magnets to bring the magnetic field effects down closer to the tube carrying the electron beam. In such arrangements it is necessary to provide for coupling an external circuit to the slow wave structure within the tube. This is generally done by waveguides which extend at right angles to the tube. These waveguides then introduce a gap in the magnetic structure and so disturb the continuity of the magnetic field introducing certain distortion.

It is an object of the present invention to provide a beam focussing device of the magnetic type which will permit the discontinuity of the magnetic field to be avoided.

According to a feature of this invention there is provided a magnetic beam focussing device made up of a plurality of magnetic units, each of these units includes two magnets with adjacent poles of opposite polarity separated by a region of high magnetic reluctance. These units are assembled with adjacent poles of opposite po-l larity facing one another. Preferably, also magnetic pole pieces are interposed between adjacent magnetic units.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which:

FIG. la is a Sectio-nal diagrammatic representation of a magnetic focussing unit of the prior art;

tFIG. 1b is a curve illustrating the variation in magnetic ux along the axis of the beam in FIG. la;

FIG. 2 is a diagram illustrating the type of disturbances which may be introduced by the structure of FIG. la;

FIG. 3a is a view, particularly in cross section, illustrating an embodiment of the present invention; and

FIG. 3b is a curve showing the variation in magnetic flux along the beam axis obtained with the structure shown in FIG. 3a.

Turning now to FIG. la, there is shown a magnetic l focussing assembly comprising a plurality of magnets 1 in the form of rings. These magnets are positioned with adjacent pole faces of like polarity. Intermediate each of the magnets are provided pole pieces of magnetic material 2. The electron beam 3 of tube 4 is shown traversing the axis of the magnet assembly. Waveguides for coupling to the external circuits are shown at 5.

By reference to FIG. 1b, it will be seen that over the greatest length of the assembly of FIG. la the ux variations along the axis follow substantially a sine wave. However, the variation departs from this sine wave characteristic along the assembly in the area in which the 3,013,172 Patented Dec. 12, 1951 coupling waveguides 5 cause interruption of the uniform magnetic structure.

Turning to FIG. 2, the nature of the disturbance for the beam can be better understood. In this figure the beam penetrating factor of the magnetic field is plotted as ordinates and the operating voltage for the electron beam as abscissa. For the greater part of the length of the magnetic element, that is, where the sine wave variation is maintained, a stable penetrating factor is obtained as shown by curve 6. The penetrating factor increases smoothly to nearly penetration beyond the area 7, which illustrates the lower limiting voltage for operation of the beam tube. However, in the areas of disturbance, represented by the waveguides 5, there is produced an excessive variation in the penetration factor with the changes in operating tube voltage as illustrated in the curve 8L These disturbing effects are undesirable. Methods of reducing or avoiding these eects have been proposed by the addition of other magnetic devices external of the waveguide 5. These additional structures, however, are difficult to construct and are quite critical, requiring extreme care in accurately adjusting them.

According to the present invention, the difficulties outlined above with respect to the prior art may be overcome by the novel magnetic focussing structure. An example of this new structure is illustrated in FIG. 3a. There are provided individual magnetic units composed of separate permanent magnets 1 and 1" separated by a spacing of high magnetic reluctance 9. This spacer may preferably be in the formy of some non-magnetic material in the interest of the construction of a unitary assembly. Magnets 1 and 1" are arranged in magnetic aiding relation. That is, with adjacent poles being of opposite polarity. It will, therefore, be apparent that at each end of these units there will be poles of opposite polarity.. A plurality of these units are then assembled with poles of opposite polarity on the separate units, adjacent one another. Intermediate these units are provided magnetic pole pieces 2 preferably made of soft iron or some such material. The permanent magnets may be made of some high magnet steels, although in the preferred construction some of the permanent magnet type of ferrites are used.

With this type of magnetic assembly, it will be apparent that the magnetic field in the axis of the assembly will not be a pure sine wave but will have ripples at the peaks of the waves producing a field along these axes corresponding to a sine wave variation with a superimposed third harmonic as shown in FIG. 3a by curve 10. It is possible to have the magnetic flux density B2 in the form shown by selection of the desired magnetic material, controlling the dimension ratio of the magnets and the spacing, and adjusting the inner diameters of the pole pieces. The magnetic flux B2 can then be represented by the following equation:

B2=cos @L- eos S22-Z where L is the length of the magnetic unit including elements 1', 1" and 9, and Z is the magnetic strength of the field at the axis. It has been found that a field closely following this equation may be determined theoreti/:ally or experimentally, having in mind the various parameters of the elements forming the assembly.

For example, magnets 1' and 1" may be made of permanent magnet ferrites with an outside diameter of 0.89 L, an inside diameter of 0.27 L, and a space of 0.18 L. Such a structure has been found to obtain a flux distribution substantially as described above.

In the application of this invention, the spacing by the elements 1 and 1" may be so chosen that the waveguide width is substantially the same as this spacing. The waveguide couplers may then be inserted between two magnets of a single unit in place of the normal spacers 9.

Accordingly with this type of construction, there will be vno discontinuity in the magnetic eld at the points at which the external couplings are applied.

In an arrangement utilizing ferrite magnets of the dimensions given above, it is found that substantially the same effect can be obtained using a spacing between the magnets of 8 mm. as is produced in a prior art structure such as shown in FIG. la with a magnetic unit of In accordance with this invention, all ofthe magnets l 1 and l" may be uniform inldimension and vmagnetization, and the assembly can, therefore, be readily made. It is quite easy, therefore, to construct focussing devices in accordance with this invention with the .desired characteristics.

While l have described above the principles `of my invention in connection with specic apparatus, it is to beclearly understood that this 4description .is made only of opposite polarity separated by'a non-magnetic material extending over a fixed region of high magnetic reluctance, whereby poles o-f opposite polarity exist at opposite ends of said unit, annular pole pieces of magnetic material positioned `between the poles of adjacent `magnetic units andlsaid units `being positioned in tandem so that theiradjacent poles are of like polarity whereby a periodic lield is provided along the-longitudinal'axis of said magnetic units. n

2. A beam focussing device according to claim 1 wherein said Vpair of magnets are each in the form of a hollow cylinder.

3. A beam focussing device according to claim,2 further comprising electrical wave transmitting devices extending from Athe external surface of one of said units into the hollow of the cylinder defined by said unit through said fixed region.

References Cited'in the jleof this patent v rUNITED 'STATES PATENTS 2,797,360 

